A typical example of coating apparatuses for coating a film-like coating liquid on a web is a curtain coating apparatus, in which a curtain of one or more coating liquids is created and is caused by gravity to impinge on an article to be coated so that a coating film is formed on the article. Such apparatus has been used from old times to coat furniture, a sheet of iron and so on. As demand for an improved coating quality has increased, so has the requirement for high precision coating. In recent years, the curtain coating apparatus has been applied to a field requiring a particular precision, such as in the manufacture of a photographic photosensitive material, as disclosed in U.S. Pat. Nos. 3,508,947 and 3,632,374.
U.S. Pat. No. 3,632,374 discloses a method of stably forming a curtain film to provide a small coating amount. In this method, perforated pipes 15 and 15' are incorporated respectively in edge guides 14 and 14' shown in FIG. 1, and an inert liquid such as a transparent gelatin solution is fed under pressure into the pipes from their upper ends so that the liquid exudes from axial slots formed in the edge guides.
However, with such an edge guide device, even if the auxiliary liquid is exuded in the direction of the width of the curtain film, any force is exerted on the coating liquid in its gravitating direction. This results in formation of a thinned film in the vicinity of each edge guide, thus prohibiting satisfactory stable formation of the curtain film.
Japanese Patent Publication No. 58-37866 describes a method in which in order to enhance the stabilization in providing a small coating amount opposite edges of a coating liquid curtain are supported by jets of a liquid in the direction of flow of the coating liquid. More specifically as shown in FIGS. 2 and 3, discharge ports 36 and 36' are provided at a lower portion of the curtain film 32 to discharge jets of an auxiliary liquid in the direction of flow of the coating liquid, to prevent the contracted flow of the curtain film at a region extending from lower ends of edge guides 24 and 24' to a surface of a web 22.
The just-described method has the advantage that the contracted flow of the curtain film in the vicinity of the lower ends of the edge guides is prevented thereby providing a uniform coated film having no locally-thickened portion. However, where the curtain is relatively tall, the coating liquid has sometimes been formed into a thinned film before it reaches the end of the discharge side, thus affecting uniformity.
It is considered that the formation of such a thinned film in the vicinity of the edge guides results from a so-called Marangoni effect, described in British patent application No. 2021001 A. It is surmised that since the surface concentration of the surface active component in the coating liquid depends on the rate of surface expansion, a distribution of the surface expansion rate caused by a velocity distribution near the edge guides gives rise to a difference of the surface active component in surface concentration that is, a difference in surface tension, so that the coating liquid is subjected to a force tending to attract it toward the portion of a higher surface tension in the curtain film.